We investigated the time dependent effects of 8 Gy of gamma radiation on cyclin-dependent kinases (Cdk's) and incorporation of BrdU through the S phase of Chinese hamster ovary (CHO) cells. The in vitro Cdk activities of immunoprecipitated cyclin E, cyclin A , cyclin E-Cdk2 and cyclin A-Cdk2 were determined over a 1.5-4.0 h periods. Results obtained indicated that the transient G1-phase delay or G1/S phase check point is mediated by inhibition of cyclin E-Cdk2 and point to the existence of a radiation induced S phase checkpoint located about 75% into S phase involving the inhibition of cyclin A-Cdk2 by a p53/Waf1-independent pathway in CHO cells.